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Question Video: Calculating the Percentage of Water of Crystallization Given the Mass of the Hydrated and Dehydrated Salt Chemistry

A 0.3548 g sample of a hydrated salt was strongly heated until a constant mass of 0.3015 g was obtained. What is the percentage of water of crystallization in this hydrated salt? Give your answer to two decimal places.

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Video Transcript

A 0.3548-grams sample of a hydrated salt was strongly heated until a constant mass of 0.3015 grams was obtained. What is the percentage of water of crystallization in this hydrated salt? Give your answer to two decimal places.

A hydrated salt is a substance that contains water of crystallization. Water of crystallization is the presence of water molecules within the structure of a crystal. For example, the hydrated salt copper(II) sulfate pentahydrate has the chemical formula CuSO4⋅5H2O. In the formula, we can see that for each unit of copper(II) sulfate, there are five molecules of water associated with it.

One way to determine the amount of water of crystallization in a hydrated salt is to use volatilization gravimetry. Volatilization gravimetry is a mass analysis method that uses thermal or chemical energy to separate substances in order to measure the masses of their components. In the question, we are told that the hydrated salt is heated until a constant mass is achieved. This could be done by heating the hydrated salt in a crucible using a Bunsen burner. As the hydrated salt is heated, water molecules are liberated. When the mass of the sample stops changing, all of the water has been removed. The water that has been removed is the water of crystallization, and what remains in the crucible is called the anhydrous salt.

To find the mass of the water lost, let’s take the mass of the sample before heating and subtract the mass of the sample after heating. After substituting the values provided in the problem, we get 0.3548 grams minus 0.3015 grams. Therefore, the mass of water lost during heating is 0.0533 grams.

Next, to find the percentage of water of crystallization in the hydrated salt, we need to take the mass of water of crystallization, which is the mass of water lost, and divide it by the mass of the hydrated compound before heating. Then, we need to multiply by 100 percent. Let’s divide 0.0533 grams, which is the mass of water lost, by 0.3548 grams, which is the mass of the sample before heating, and multiply by 100 percent. The result is approximately 15.0225 percent.

Finally, the question asks us to give our answer to two decimal places. Therefore, the percentage of water of crystallization in the hydrated salt is 15.02 percent.

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